The cardiac pacemaker in its simplest form is an electrical circuit in which a battery provides electricity that travels through a conducting wire through the myocardium, stimulating the heart to beat (“capturing” the heart), and back to the battery, thus completing the circuit. Implantable cardiac pacemakers have been in existence since the later 1950's, although external pacemakers were known even earlier. Since that time great strides have been made in improving upon the leads and the pulse generators that together comprise the pacemaker. In particular, the pulse generator circuitry has evolved from discrete components to semi-custom integrated circuits, which are now fabricated from complimentary metal oxide semi-conductor (CMOS) technology.
As cardiac pacemakers have evolved they have been designed to provide increases in the heart rate for periods when the patient is experiencing physiological stress. These “rate-modulating” pacemakers help the patient adapt to physiological stress with an increase in heart rate, even if the patient's intrinsic heart rate would not allow this to occur. The development of dual-chamber pacemakers has allowed the patient to increase their heart rate if he or she is in sinus rhythm.
The rate-modulated pacemaker has three major components. The first is an indicator, such as for activity, body temperature, or respiratory rate, that provides an approximate measurement of metabolic needs. The second is a sensor that can measure the indicator chosen, such as measurement of body temperature or respiratory rate. The third is a rate controlled algorithm that is in the software of the pacemaker and modulates the pacemaker rate as the sensors send signals to the pacemaker.
As the sensors indicate greater metabolic need, the pacing rate is increased. The rate at which the pacing rate changes, however, is bounded and controlled by a feature called rate smoothing. Rate smoothing is a gradual slowing or speeding of the pacemaker rate based on a percentage of a preceding cardiac interval. This is a mechanism programmed into such types of pacemakers to reduce or to smooth abrupt changes in paced rate, especially at the upper rate limit of dual chamber pacemakers. Conversely, if a patient were to develop an ectopic atrial tachycardia, this programmed feature would cause a gradual increase in rate rather than an abrupt increase in rate.
Rate smoothing may, however, unnecessarily limit a heart rate change under some circumstances. For example, when an individual needs rapid cardiac output in a short time, such as in a stressful situation, rate smoothing may prevent the heart rate from rising rapidly enough to keep up with the individual's cardiac demands. Similarly, once the stressful situation has passed, the individual's pacing rate will decrease under the constraints imposed by the rate smoothing algorithm. Furthermore, the present inventors have also recognized that rate smoothing may interfere with arrhythmia prevention and treatment by limiting a change in heart rate that is needed to prevent or treat the arrhythmia. Thus, there exists an unmet need in the art for more flexible rate smoothing.